mcp: wrap unmarshalParams with jsonrpc error (#1087)

Fixes #976

docs/mcpgodebug.md

@@ -50,6 +50,13 @@ Options listed below were added and will be removed in the 1.9.0 version of the
   restoring the previous behavior. The default behavior was changed so that
   the client always attempts to surface the underlying JSON-RPC error.
 
+- `nowrapinvalidparams` added. If set to `1`, the server will not wrap
+  params-decoding failures with `jsonrpc2.ErrInvalidParams`, so wire responses
+  carry the zero-value error code `0` instead of `-32602` ("invalid params"),
+  restoring the previous behavior. The default behavior was changed so that
+  the server always reports params-decoding failures with the standard
+  `-32602` code.
+
 - `disablecompleteparamsvalidation` added. If set to `1`, `Server.complete`
   will not validate that the required `ref` and `argument.name` fields on
   `CompleteParams` are present, restoring the previous behavior of dispatching

internal/docs/mcpgodebug.src.md

@@ -49,6 +49,13 @@ Options listed below were added and will be removed in the 1.9.0 version of the
   restoring the previous behavior. The default behavior was changed so that
   the client always attempts to surface the underlying JSON-RPC error.
 
+- `nowrapinvalidparams` added. If set to `1`, the server will not wrap
+  params-decoding failures with `jsonrpc2.ErrInvalidParams`, so wire responses
+  carry the zero-value error code `0` instead of `-32602` ("invalid params"),
+  restoring the previous behavior. The default behavior was changed so that
+  the server always reports params-decoding failures with the standard
+  `-32602` code.
+
 - `disablecompleteparamsvalidation` added. If set to `1`, `Server.complete`
   will not validate that the required `ref` and `argument.name` fields on
   `CompleteParams` are present, restoring the previous behavior of dispatching

mcp/shared.go

@@ -26,9 +26,21 @@ import (
 	"github.com/modelcontextprotocol/go-sdk/auth"
 	internaljson "github.com/modelcontextprotocol/go-sdk/internal/json"
 	"github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2"
+	"github.com/modelcontextprotocol/go-sdk/internal/mcpgodebug"
 	"github.com/modelcontextprotocol/go-sdk/jsonrpc"
 )
 
+// nowrapinvalidparams is a compatibility parameter that restores the previous
+// behavior of [methodInfo.unmarshalParams]. When unset (the default), a
+// params-decoding failure is wrapped with [jsonrpc2.ErrInvalidParams] so the
+// wire response carries error code -32602 ("invalid params") rather than the
+// zero-value code 0. See:
+// https://github.com/modelcontextprotocol/go-sdk/issues/976#issuecomment-4829124838.
+//
+// See the documentation for the mcpgodebug package for instructions how to enable it.
+// The option will be removed in a future version of the SDK.
+var nowrapinvalidparams = mcpgodebug.Value("nowrapinvalidparams")
+
 const (
 	// latestProtocolVersion is the latest protocol version that this version of
 	// the SDK supports.
@@ -304,7 +316,17 @@ func newMethodInfo[P paramsPtr[T], R Result, T any](flags methodFlags) methodInf
 			var p P
 			if m != nil {
 				if err := internaljson.Unmarshal(m, &p); err != nil {
-					return nil, fmt.Errorf("unmarshaling %q into a %T: %w", m, p, err)
+					// Legacy behavior: pre-fix versions surfaced this as a
+					// plain wrapped error, which caused the wire response to
+					// carry code 0 instead of -32602. Restore via
+					// MCPGODEBUG=nowrapinvalidparams=1.
+					if nowrapinvalidparams == "1" {
+						return nil, fmt.Errorf("unmarshaling %q into a %T: %w", m, p, err)
+					}
+					// Wrap jsonrpc2.ErrInvalidParams so toWireError surfaces
+					// code -32602 ("invalid params") while preserving the
+					// descriptive message.
+					return nil, fmt.Errorf("%w: unmarshaling %q into a %T: %w", jsonrpc2.ErrInvalidParams, m, p, err)
 				}
 			}
 			// We must check missingParamsOK here, in addition to checkRequest, to

mcp/shared_test.go

@@ -512,3 +512,29 @@ func TestImplementationDescriptionJSON(t *testing.T) {
 // 		}
 // 	})
 // }
+
+// TestUnmarshalParamsInvalidParamsCode is a regression test for
+// https://github.com/modelcontextprotocol/go-sdk/issues/976#issuecomment-4829124838.
+// Malformed params for a registered method must surface as a JSON-RPC error
+// whose code is CodeInvalidParams (-32602) rather than the zero-value 0.
+func TestUnmarshalParamsInvalidParamsCode(t *testing.T) {
+	info, ok := serverMethodInfos[methodPing]
+	if !ok {
+		t.Fatalf("no methodInfo for %q", methodPing)
+	}
+	// Array where a struct is expected.
+	_, err := info.unmarshalParams(json.RawMessage(`["a","b"]`))
+	if err == nil {
+		t.Fatal("unmarshalParams returned nil error for malformed params")
+	}
+	var jerr *jsonrpc.Error
+	if !errors.As(err, &jerr) {
+		t.Fatalf("expected *jsonrpc.Error in chain, got %T: %v", err, err)
+	}
+	if jerr.Code != jsonrpc.CodeInvalidParams {
+		t.Errorf("error code = %d, want %d (CodeInvalidParams)", jerr.Code, jsonrpc.CodeInvalidParams)
+	}
+	if !strings.Contains(err.Error(), "unmarshaling") {
+		t.Errorf("error message = %q, want it to mention unmarshaling", err.Error())
+	}
+}